1. Field of the Invention
The present invention relates in general to sound attenuating systems, and more particularly to sound attenuating systems on a type which ms designed to have both passive and active acoustic reflecting surfaces for achieving appropriate sound attenuation.
2. Description of the Prior Art
Hitherto, for reducing noise propagating in a given conduit, various passive type silencers have been proposed and put into practical use, which are of an expansion type, resonance type, interference type, acoustic absorption type and the like. In these passive type silencers, within a conduit in which noise propagates, there is provided a surface of discontinuity of acoustic impedance to form an acoustic reflecting surface by which a part of acoustic energy is reflected toward a sound source. Furthermore, in such silencers, noise suppression is achieved by using an interference of the acoustic wave in the conduit.
Recently, a so-called "active noise control" which uses active acoustic conductance has been developed. In this control, a noise or sound (primary sound) which is to be attenuated is overlapped with another sound (secondary sound) which has a phase opposite to that of the primary sound. This control is disclosed in U.S. Pat. No. 2 043 416 granted in 1934 to P Lueg. If the method disclosed by this patent is carried out theoretically, a perfect noise reflecting surface should be produced for a broad band of sound frequency to achieve a perfect noise suppression.
Among the active noise control, widely used currently for noise attenuation in a conduit system are the methods which are based on the measure disclosed by the above U.S. Patent. That is, the sound pressure of a primary sound at a secondary sound source (viz., loudspeaker) is calculated by carrying out a digital signal treatment on a reference signal issued from a sound detector (viz., microphone) located upstream of the secondary sound source. By using the sound pressure thus calculated, a secondary sound having a phase opposite to that of the primary sound is produced for cancellation of the primary sound.
Theoretically, a perfect cancellation of the primary sound is obtained when the secondary sound has an amplitude equal to that of the primary sound and a phase properly reversed to that of the primary sound, that is, when a perfect sound reflecting surface is produced on the secondary sound source. This means a formation of an acoustic wave blocking surface at the secondary sound source for the perfect cancellation of the primary sound. That is, theoretically, when the perfect sound reflecting surface is produced, propagation of acoustic wave is not permitted in a region downstream of the sound reflecting surface. Accordingly, in this theoretical case, the secondary sound source is only the element which is to be taken into consideration.
In addition to the above-mentioned active control method, a so-called "Tight-Coupled Monopole Method" was proposed by K. H. Enhtesadl et al in 1983. In this method, a microphone nor detecting a reference signal and a loudspeaker for serving as a secondary sound source are arranged at the same position. This method is advantageous in that the method is hardly affected by disturbance of the acoustic wave propagation in the conduit system and can bring about production of a sound attenuating device which is simple in construction. A similar method was proposed by Olsen in 1953.
However, the methods of the above-mentioned active noise control have the following drawbacks due to their nature.
First, the frequency band which permits formation of the perfect noise reflecting surface is not sufficiently broad and the acoustic absorption is poor. This means that there is inevitably created a frequency band within which a negative noise reduction (viz., noise increase) appears. Furthermore, if a sufficient sound attenuating effect is intended, the arrangement of the various elements in the conduit system becomes complicated and thus pressure loss in the conduit system becomes marked.
Second, as is described hereinabove, formation of the perfect noise reflecting surface is possible only in theory. From a practical point of view, production of a real device employing such theory is impossible. That is, due to interference of a reflected acoustic wave at a downstream side of a secondary sound source and of an acoustic wave which has been reflected from an upstream side of the conduit system, the device fails to exhibit a satisfied acoustic attenuation.
Third, usually, in the conventional active noise control methods, the reference signal used for deriving the sound pressure of the primary sound at the secondary sound source is detected by a microphone which is positioned away from the secondary sound source. However, as is known, the propagation characteristic of the acoustic wave varies in accordance with the temperature of gas in the conduit system and the velocity at which the gas flows therethrough. This characteristic change causes production of an error which appears when a deviation of the secondary sound for cancelling the primary sound is carried out. Hitherto, adaptive signal processing has been used for dealing with this undesired matter. However, even this processing can not deal with a rapid change of the gas temperature and the gas velocity. Furthermore, the electric system for treating the signals becomes complicated.
Fourth, as is described hereinabove, the active noise control using the "Tight-Coupled Monopole Method" is the method which aims at formation of the perfect noise reflecting surface. Accordingly, in this control, it is necessary to reverse the phase of an acoustic signal detected by a microphone and infinitely amplify the signal by using an amplifier with an infinite gain. However, in practical use, due to the nature of the phase characteristic, undesired oscillation is inevitably produced. Accordingly, the formation of the perfect noise reflecting surface is not realized.
The above-mentioned matters to which the conventional sound attenuating systems are subjected will be itemized in the following.
(1) In the passive type silencers, the arrangement of elements in the conduit system is complicated and thus the pressure loss in the system becomes marked.
(2) The passive type silencers for attenuating low frequency acoustic waves have an inevitably bulky construction.
(3) When applied to wide band acoustic wave, the passive type silencers are forced to produce a band in which negative noise reduction (viz., noise increase) appears.
(4) The active type silencers hitherto proposed are those which aim at formation of the perfect noise reflecting surface. However, the formation of such a surface is impossible in practical. Due to interference of various elements in the conduit system, including a terminal end of the conduit system, such silencers fail to exhibit a satisfaction acoustic attenuating performance.
(5) In active type silencers of a type in which with a microphone for detecting a reference signal and a loudspeaker for producing a secondary sound are positioned away from each other in the main conduit, the propagation characteristic of the acoustic wave varies in accordance with the temperature of gas in the conduit system and the velocity at which the gas flows therethrough. In order to deal with this characteristic change, adaptive signal processing has been used. However, this processing requires a very complicated electronic system and thus it is very difficult to put this system into practical use.
(6) The "Tight-Coupled Monopole Method" can solve the drawback mentioned in the item (5). However, due to its nature, undesired oscillation tends to occur.
(7) In the active type silencers of the type wherein active elements are arranged in a main conduit of the conduit system, it is necessary to use a material which can bear the gas flowing through the main conduit of the system. That is, in the conduit system through which high temperature gas and/or corrosive gas flows, conventional microphones and speakers can not be used.